Soil Health Series | Soil Structure

Over the next few weeks we will be posting an article about the pillars of healthy soils and crops and the foundation of food production in modern agriculture. Each area is as important as the other and while we often focus on one or two if these pillars (often Mineral Fertilisers and Soil Structure) we neglect the others which have the result of degrading our soils and restricting performance.

  1. Building Humus
  2. Soil Microbes
  3. Soil Structure
  4. Mineral Fertilisers

Soil Structure

Many people are aware of what soil type they are farming on but few consider a soil’s structure, though, even though in most soils, the structure is just as important. Two of the same types of soils can perform very differently depending on their structure.

The structure of a soil is one of the most important factors in determining soil health and therefore the productivity of a paddock. Often however this is overlooked as a defining factor in crop success. Your soil structure will influence the drainage and water holding capacity of the soil as well as other crucial factors including the aeration available in the soil. Soil structure also has a big physical impact on plants by influencing root growth and penetration and aiding the flow of nutrients.

What we look for in a soil is a good aggregate or friable (crumbly) soil. This has multiple benefits as there will be good moisture holding capacity within the soil crumbs and also around the crumbs, there is plenty of drainage and aeration to allow roots to thrive. Other soil organisms including microbes and earthworms will also benefit from a good soil structure as oxygen is available while moisture is able to be held and movement through the soil is easier.

How soil structure develops

Soil structure refers to how particles of soil are grouped together into aggregates. They are cemented or bound together by physical, chemical, and biological processes.

Physical-chemical processes that build soil structure include:

Polyvalent cations like Ca2+, magnesium Mg2+, and aluminum Al3+ bind together clay particles

Soil particles are pushed closer together by freezing and thawing, wetting and drying, and by roots pushing through the soil as they grow in length and width.

Biological processes that build soil structure include:

Soil particles are cemented together by humus, by organic glues created by fungi and bacteria decomposing organic matter, and by polymers and sugars excreted from roots.

Fungal hyphae and fine roots stabilize aggregates (University of Minnesota Extension 2002.)

Organic matter and plant roots are therefore key to soil structure.

How soil structure deteriorates

Factors that can deteriorate or destroy soil structure include, for example:

  • Compaction
  • Cultivation
  • Removal of vegetation
  • Excessive moving and handling of soil
  • Screening

How to Enhance Soil Structure

  • Add organic matter in the form of crop residues or green manures. Soil organic matter helps bind soil particles together in crumbs and also directly improves the moisture holding capacity of soil.
  • Introduce pasture plants with deep or fibrous roots to increase porosity and add organic material that helps bind soil particles together. In compacted or clay soils, spreading roots break up soil and create pathways through which water can seep deep within the subsoil. Decomposing root masses provide organic material that soaks up water like a sponge. In dry, sandy or rocky soils, organic material helps hold soil together and increases water retention.
  • Add lime - Calcium availability helps flocculation (binding aggregates through holding colloids, humus and clay particles, together). Calcium is important for good soil structure as this double charged cation has the effect of flocculating or drawing together soil particles to help form a nice crumb structure in soil (bringing good drainage and soil aeration).
  • Reduce stock loading to reduce the likelihood of compaction
  • Add a Microbial inoculant – an inoculant like EM will help build soil structure by stimulating the biological function in the soil. EM adds both fungu and bacteria to a soil and stimulates resident microbes to speed up the biological function of a soil to breakdown organic matter and build a positive soil structure.